Loudspeakers with a frequency response that can be adjusted to specific requirements of a listener are known within the art. Traditionally adaptation has taken place by the measurement of the sound pressure level at the particular listening position, i.e. a suitable measuring microphone is placed at the position which is to be occupied by the head of the listener and the frequency response of the loudspeaker is measured at this position. The frequency response at this position is the resulting frequency response of the loudspeaker itself (as measured in an anechoic chamber) and the acoustic effect of the particular listening room. Even if the frequency response of the loudspeaker itself is very uniform over frequency, the acoustical characteristics of the room, i.e. reflections from the boundaries of the room and from various objects located in the room, can result in a very non-uniform frequency response at the listening position, a frequency response which moreover may depend very much on the exact measuring position. Thus, corrections of the free field frequency response of the loudspeaker itself based on such measurements are not satisfactory.
Basically there are two aspects of adapting the acoustical response of a loudspeaker to a given room, which result from the following two problems:
(1) The loudspeaker's ability to provide acoustic power to the room depends on the location of the loudspeaker in the room, i.e. its position relative to the boundaries of the room. Thus, for instance when a loudspeaker is moved towards a corner position in a room, the low frequency response of the loudspeaker increases, which may lead to an undesirable “boomy” bass reproduction.(2) Even though the ability of the loudspeaker to provide acoustic power to the room may be made practically independent on frequency (or have a particularly desirable frequency dependency), the frequency response of the loudspeaker measured at a particular listening position in the room may exhibit quite large deviations from the target response due to the influence of room acoustics on the transfer function of the loudspeaker from the position of the loudspeaker to the actual listening position. It is not possible to compensate for these deviations without knowledge of the actual sound field generated by the loudspeaker at the particular listening position.
The first of the above aspects has been dealt with extensively in EP-0,772,374 and EP-1,133,896. In such systems, a digital correction filter is inserted into the signal chain. The correction filter in such systems is based on two measurements of the radiation resistance. First the radiation resistance is measured in a reference loudspeaker position in a reference room. Then the measurement is repeated in the actual loudspeaker position in the actual room, e.g. in the living room belonging to the user of the loudspeaker. (Measurements could alternatively also be performed at two different positions in the listening room, the actual position for some reason giving rise to undesirable acoustical effects and the reference position being regarded as acoustically more satisfactory). The relationship between these two measured radiation resistances then determines the characteristics of the correction filter in such a way that the perceived timbre using the actual loudspeaker position in the actual room resembles to a large extent the perceived timbre using the reference loudspeaker position in the reference room or the more satisfactory position in the actual listening room.
The above system thus adapts the loudspeaker to the actual listening room as such, but it does not compensate for the above-mentioned deviations of the frequency response from a given target at a particular listening position in the actual listening room.